Signal attenuation measurements generally involve transmitting a signal towards or through a tissue medium under analysis, detecting the signal transmitted through or reflected by the medium and computing a parameter value for the medium based on attenuation of the signal by the medium. In simultaneous signal attenuation measurement systems, multiple signals are simultaneously transmitted (i.e., two or more signals are transmitted during at least one measurement interval) to the medium and detected in order to obtain information regarding the medium.
Such attenuation measurement systems are used in various applications in various industries. For example, in the medical or health care field, optical (i.e., visible spectrum or other wavelength) signals are utilized to monitor the composition of respiratory and anesthetic gases, and to analyze tissue or a blood sample with regard to oxygen saturation (SpO2 level), analyte values (e.g., related to certain hemoglobins) or other composition related values.
The case of pulse oximetry is illustrative. Some pulse oximeters extract information regarding patient physiological conditions such as the patient's pulse rate and an oxygen saturation level of the patient's blood, or related analyte values, via analysis of plethysmographic signals or waveforms corresponding to different wavelengths of light transmitted through or reflected from the patient's tissue. In particular, pulse oximeters generally include a probe for attaching to a patient's tissue site such as a finger, earlobe, nasal septum, or foot. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, to the patient's tissue site. The different wavelengths of light used are often referred to as the channels of the pulse oximeter (e.g., the red and infrared channels). The optical signals are attenuated by the patient tissue site and subsequently are received by a detector that provides an analog electrical output signal representative of the received optical signals. The attenuated optical signals as received by the detector are often referred to as the transmitted signals. The electrical signal can be processed to obtain plethysmographic signals for each channel and the plethysmographic signals may be analyzed to obtain information regarding patient physiological conditions.
Extraction of patient physiological conditions from the plethysmographic signals can be quite effective using a well positioned sensor and when the patient or subject is resting. However motion artifacts can easily swamp the desired information included in the plethysmographic signals when the patient is moving around and/or performing muscular contractions. Some motion artifacts can severely impair the signals, whereas other types can be filtered out or do not significantly effect the desired information included in the plethysmographic signals. Furthermore, depending upon the severity and type of motion artifacts present in the plethysmographic signals, some techniques for extracting the desired patient physiological conditions may not be appropriate and alternative techniques may need to be employed. Another potential problem that can occur when attempting to make a pulse oximetry system robust to motion artifacts is that heart arrhythmia or rapid pulse variations might possibly be sensed as motion effects and cause motion rejection steps to be applied which may be inappropriate in these cases and could cause false pulse-rate and SpO2 readings.